Doug,
I'm confused by your numbers.
I'm good with the chamber measurement being different by 0.002.
I regularly adjust cartridge base to ogive measurements more than that to account for differences in trim length to match exit time.
I think you lucked out on that one with the new barrel.
However, I don't understand the reflection time measurements for the new barrel.
I thought you were ordering a 27-inch 416R barrel. But then again you might have changed your mind and ordered a 26 inch barrel.
As far as I am concerned, 26-inch barrels work beautifully for the 6.5mm Creedmoor.
First, the old 12 FV 3% carbon steel, 26-inch nominal barrel is actually 25.96 because it has a 0.040 recess to the crown.
If your new barrel doesn't have a recessed crown, add 0.002 msec. to the reflection time.
Add or subtract, 0.001 msec. to the exit time for each 0.020 of recess.
The Savage 12 FV barrel has a steel reflection speed of 19,107 fps so it has exit times as follows:
10th reflection = 1.1322 msec. or 1.132 if your using QuickLOAD.
12th reflection = 1.359 msec.
Each 0.002 of increased seating depth adds 0.001 msec. of exit time. Reducing seating depth by the same amount subtracts 0.001 of exit time.
For the same length 416R steel barrel with the same recessed crown with a reflection speed of 20,014 fps the exit times are:
10th reflection = 1.0809 msec. or 1.081 if your using QuickLOAD.
12th reflection = 1.297 msec.
If your new barrel is 27 inches with the same recessed crown, the 12th reflection time is 1.347 - that is 0.050 msec. more per inch of barrel length.
I have had problems trying to get most powders close to 1.132 msec. without exceeding Pmax, so I use the 12th reflection as my exit time goal.
For the Savage barrel, and the 143 Hornady ELD-X,
With H4350, exit time of 1.259 msec. is just under Pmax.
With H4831 SC, 1.247 msec. is just under Pmax.
That says the 12th reflection is safe BUT the 10th reflection is unattainable and seriously unsafe, almost 3.0 grains over the Pmax for each powder, respectively.
The pressure the charge produces doesn't change with the barrel steel composition, so the same pressure numbers from QuickLOAD are valid for both barrels.
But the exit time reduces for the 416R steel barrel and you have to increase the charge to get more velocity and smaller exit time, so the Pmax problem gets even worse with the 416R steel barrel.
Don't be too concerned with getting to the exact exit time, especially while you are learning your new barrel stock combination.
With my 12 FV with the Savage barrel, the difference of 0.002 in exit time is only 0.007 inches in group size on average which is well within my 'shooter induced variation' range, even trying my best to eliminate the variation effects.
Bookmarks